To achieve desired purposes such as improving long-term corrosion resistance of a steel sheet, plating has been used since olden days. Such a plated steel sheet has a plating layer, and a representative example of composition of the plating layer is an alloy containing 1 to 75 mass % aluminum and zinc which accounts for most of the residue, and a minute amount of the third component such as Si, Mg, and Ce—La.
The conventional plated steel sheets are excellent in corrosion resistance. However, this only means that time to occurrence of red rust resulting from corrosion of a base steel sheet is long. If the surface of the plating is not subjected to any coating treatment, white rust or blackening may occur at a short time, and as a result excellent appearance of the plated steel sheet may be lost. In recent years, when the plated steel sheet is used as a building component, the surface of the plating layer is prone to tarnish over time due to acid rain.
Therefore, techniques involved in surface treatment to suppress blackening of the plated steel sheet have been proposed recently. In particular, many techniques of chromium-free treatment using a surface treatment agent free of hexavalent chromium have been used, and many of these techniques are disclosed.
For example, Patent document 1 discloses a chromium-free surface treated plated steel sheet. The chromium-free surface treated plated steel sheet includes a hot-dip Zn—Al based alloy plated steel sheet and a coating film formed on the hot-dip Zn—Al based alloy plated steel sheet. The melted Zn—Al based alloy plated steel sheet has a melted Zn—Al alloy plating layer containing 1.0% to 10% Al, 0.2% to 1.0% Mg, and 0.005% to 0.1% Ni. The coating film is made of a treatment composition containing: a specific titanium-containing aqueous solution; and a nickel compound and/or a cobalt compound; and a fluorine compound; and an aqueous organic resin, at predetermined ratios. Patent document 1 also discloses that the chromium-free surface treated plated steel sheet is excellent in corrosion resistance, blackening resistance, coating adhesion, and external appearance of the plate.
However, performance offered by the techniques disclosed in Patent document 1 is practically insufficient in corrosion resistance, blackening resistance, acid resistance, and formability in view of recent environmental circumstances, and sufficient techniques have not been obtained yet, actually.